The present invention relates to a method and apparatus for detecting an open circuit condition of a thermocouple.
A thermocouple is a bi-metallic junction device for electrically measuring temperature. The bi-metallic junction of the thermocouple produces a voltage across the junction in accordance with the temperature of the thermocouple, wherein a voltmeter may be used to read and record the voltage so as to monitor temperature. In normal operation, the thermocouple resistance is quite low. However, if the two metals associated with the bi-metallic junction lose contact with one another, then an open circuit failure condition results and the thermocouple will no longer produce the desired voltage.
In measurement applications, a thermocouple may be inserted into a system for monitoring the temperature of a given element for assisting the system in regulating the temperature. The voltage produced by the thermocouple is read by an instrument for detecting when the temperature of the element exceeds a predetermined threshold, and corrective action may follow. However, if the thermocouple fails, the system may also fail. It is therefore desirable to periodically check the thermocouple for an open circuit condition in order to verify whether the device is functional.
One known method for detecting a failed open circuit thermocouple is illustrated in FIG. 1. A square wave signal received at input 1 is applied to thermocouple 5 via coupling capacitor 2. The source of the square wave signal has a source impedance Z.sub.s. The source impedance and the resistance of thermocouple 5 provide a resistor network for dividing the available magnitude of the input signal and producing a signal at input 1. When thermocouple 5 is operating normally and its resistance is accordingly low, the associated divider network produces a low magnitude for the divided signal. In contrast, when the thermocouple fails and becomes an open circuit, the magnitude of the divided signal increases. Diode 3 is disposed in series between input 1 and comparator 6, its anode being connected to input 1 and its cathode being connected to the positive input of comparator 6. Shunting capacitor 4 is also connected to the plus input of comparator 6 and provides a shunt filtering capacitor for filtering the signal as rectified by diode 3. The negative input of comparator 6 is connected to reference voltage V.sub.r. When the thermocouple is operating normally with low resistance, the peak detecting circuit comprising rectifying diode 3 and capacitor 4 generates a low output voltage generally corresponding to the magnitude of the square wave available at the input 1.
The value of reference voltage V.sub.r is set to be greater than the filtered rectified voltage when the thermocouple is operating normally, but less than the filtered rectified voltage when the thermocouple has failed to an open circuit condition. Upon the occurrence of such failure, the magnitude of the divided signal increases and the filtered rectified voltage becomes greater than the reference voltage V.sub.r whereby comparator 6 reports the open circuit condition by transitioning V.sub.out from a low voltage to a high voltage. One of the disadvantages of this prior art circuit is its susceptibility to noise. With thermocouple 5 in a noisy environment, it electrically couples noise from the environment to the peak detection circuitry which may trip and cause comparator 6 to report an erroneous failure condition.
Another prior art circuit is shown in FIG. 2 wherein a small current pulse is applied through resistor 11 to thermocouple 5 which provides a voltage pulse in response thereto. A/D circuit 12 digitizes the voltage pulse and produces digital signal D.sub.out. By processing the digital signal to determine a pulse amplitude change, the presence of an open circuit thermocouple may be determined. In operation, a control signal periodically closes switch 10 for applying current to the thermocouple. When the thermocouple is operating normally, its resistance is low and the resulting voltage pulse produced has a low amplitude. In contrast, when the thermocouple is in an open circuit condition, the voltage pulse has a large amplitude and the change in pulse amplitude can be detected and reported. A problem with this prior art circuit is that when an external instrument, such as a strip chart recorder 14, is attached across the thermocouple, the instrument may receive and record undesirable artifact glitches corresponding to the applied pulses.
Another prior art circuit is illustrated in FIG. 3 wherein small voltage pulses V.sub.in are applied to input 1 for producing an output voltage V.sub.out, wherein the average value of V.sub.out depends on the thermocouple's condition. In one path, V.sub.in is coupled in series through capacitor 20, diode 26 and resistor 27 to thermocouple 5. In a second path, the input is applied through diode 21 to V.sub.h -limiting diode 22, as well as through shunting capacitor 24 and resistor 28 to thermocouple 5. Sensing node 25, joining the cathode of diode 21, the anode of diode 22, shunting capacitor 24 and resistor 28, carries a sensing voltage in accordance with the accumulation of charge across shunting capacitor 24. In a third path, the input voltage is applied, via timing block 23, to the gate of integrating/sampling FET 29 for periodically sampling the sensor voltage.
In operation of the FIG. 3 circuit, the shunting capacitor 24 acts as an integrating capacitor and accumulates charge with each leading edge of the input pulse V.sub.in. Following each leading edge, the capacitor discharges accumulated charge according to an RC time constant associated with capacitor 24, resistor 28 and the resistance of thermocouple 5 (ignoring the resistance of FET 29 when the FET is off and ignoring the resistance of a voltage measurement device when the FET is on). When thermocouple 5 is operating normally, the integrating capacitor discharges rapidly between leading edges through a low resistance discharge path comprising resistor 28 and the low resistance of thermocouple 5, thus providing a low sense voltage when sampled. On the other hand if thermocouple 5 fails in an open circuit condition, the discharge path resistance becomes high and the integrating capacitor accumulates charge with each leading pulse edge with no discharge therebetween until the sense voltage limits at V.sub.h. Once the voltage level at node 25 exceeds a predetermined threshold, an open circuit condition is detected. A problem with this circuit is that it requires several input pulses before the integrating capacitor accumulates enough charge for raising the sense voltage to the threshold voltage, i.e., it is slow.
In a prior art circuit as illustrated in FIG. 4a, V.sub.comp is a DC voltage when thermocouple 5 is operating normally and a square wave when the thermocouple has failed to an open circuit condition. A square wave signal V.sub.in is received at input 1 and applied to resistors 31 and 35. The opposite side of resistor 31 is coupled to the negative input of comparator 39 and also to capacitor 32 disposed in series with resistor 33 and thermocouple 5. The terminal of resistor 35 opposite input 1 is attached to the positive input of comparator 39 as well as to shunting capacitor 36. A small bias voltage V.sub.bias+ is supplied through resistor 38 to the positive input of comparator 39.
With reference to FIG. 4b, when the resistance of the thermocouple is low, i.e. operating normally, the feedback voltage V.sub.fb provided at the minus input of comparator 39 is always lower than the reference voltage V.sub.r provided at the plus input of the comparator. V.sub.fb at the negative input of comparator 39 is a square wave having an amplitude comparable to V.sub.r at the positive input but offset just below V.sub.r, the latter being provided with a slight DC offset as a result of V.sub.bias. Consequently, the comparator's output V.sub.comp is a DC output represented by curve 54.
When the thermocouple fails, V.sub.comp becomes a square wave. FIG. 4b shows that at the time of failure, T.sub.f, the feedback voltage V.sub.fb, 53, increases in magnitude for providing the negative terminal of the comparator with a voltage greater than that at the positive input for changing V.sub.comp to a square wave. Low pass filter 34 filters V.sub.comp and drives a voltage comparator 37 with its average DC voltage. Comparator 37 then determines when the filtered voltage drops below given threshold V.sub.th and signals an open circuit condition.
A problem with the FIG. 4a circuit occurs as a result of an amplitude/noise compromise. If the amplitude of the input voltage is low, it is possible for the thermocouple to pick up extraneous noise causing comparator 37 to signal an erroneous open circuit detection. On the other hand, if the amplitude of the input is too high (for overcoming noise problems), externally attached instruments, i.e. such as a strip chart attached to the thermocouple, may observe the applied input square wave and record respective artifact glitches.
A derivative prior art circuit of FIG. 4a uses synchronous detection, wherein the low-pass filter of FIG. 4a is replaced with a synchronous detector. Rather than low-pass filtering the square wave for DC and detecting when the average voltage shifts, a synchronous detector determines the presence of the square wave once the thermocouple fails.